1. Field of the Invention
The present invention relates generally to the art of lead-acid batteries, and more particularly to lead-acid batteries of the absorptive mat variety. More specifically, the present invention relates to improved thermal management of such batteries.
2. Description of the Prior Art
Sealed lead-acid batteries in which oxygen gas generated at the positive electrode during charging of the battery is absorbed by the negative electrode have previously been known in two general categories. One is a retainer type and the other is a gel type. Both are generally considered to be of the acid-starved variety. This specification relates to batteries of the former type, in which a separator (typically a mat separator) is inserted between the positive and negative plates to isolate the electrodes and to retain the acid electrolyte. Sealed lead-acid batteries utilizing a gel, typically made from sulfuric acid electrolyte and a gelling agent, offer an alternative, but typically gelled systems are inferior discharge capacity. Examples of such batteries are shown in U.S. Pat. No. 3,776,779, issued Dec. 4, 1973 to Johnson for "Gelled Battery Electrolyte Containing a Polyglycol Polymer and a Process for Locating Same Within a Lead-Acid Cell"; U.S. Pat. No. 5,035,966, issued Jul. 30, 1991 to Tokunaga, et al. and entitled "Sealed Lead-Acid Battery"; U.S. Pat. No. 3,930,881, issued Jan. 6, 1976 to Cestaro, et al. and entitled "Immobilized Battery Electrolyte"; and U.S. Pat. No. 4,414,302, issued Nov. 8, 1983 to Jache, et al. and entitled "Method of Making a Lead Storage Battery and Lead Storage Battery Made According to this Method." This group of patents relates primarily to techniques for improving the gel used in such systems or the performance of the battery resulting therefrom.
In the mat systems described to this point, a space will exist around the periphery of the electrode/separator bundle, just inside the inner wall of the container. Such space is an air space and is hence insulating. It has been found in recent work that high internal temperatures generated during battery use tend to deteriorate battery performance over time. Thermal management is therefore highly desirable. It would be desirable in such absorptive mat systems to dissipate heat from the battery electrode/separator elements to prolong battery life and efficiency.
Several glass mat batteries known in the art are represented, for example, by U.S. Pat. No. 5,091,275, issued Feb. 25, 1992, to Brecht, et al. for "Glass Fiber Separator and Method of Making." This patent will be described here by way of background. The particular invention disclosed in this patent is a particular mat which includes micro-fibers and a binder, the latter including an aqueous mixture of colloidal silica particles and a sulfate salt. When the liquid electrolyte is added to this battery, it is wholly absorbed into the elements.
As discussed in this patent, lead-acid batteries typically include a plurality of electrode plates with separators disposed between electrode pairs to prevent metallic deposits in the battery from forming short circuits. Typically, the separators are porous so that electrolyte is absorbed therein to permit current to pass from one plate to another.
The patent also discloses the reactions which take place at the positive electrode, namely the hydrolysis of water, producing oxygen. Oxygen can react with the lead of the negative electrode to produce lead oxide, which is thereafter reduced to metallic lead, liberating the oxygen which then reforms water. Oxygen recombination, as is known, is limited by the rate of oxygen transport from the positive electrode to the negative electrode. The patent goes on to point out that certain silica materials establish desirable oxygen concentration gradients that drive the transport of oxygen along silica chains from the positive to the negative electrodes. Prior to the invention discussed in this patent, absorptive glass mat separators had been known, but complex equipment was needed for placing and compressing a separator between the electrodes. This patent describes one particular operation in which the absorptive glass mat can be handled in a rigid, compressed state and will expand with the addition of battery electrolyte to provide the desired separator-electrode plate contact. The way in which this is done is to mix a sulfate salt with colloidal silica to produce a dried mat material which can then be more easily handled during battery assembly. The colloidal silica used in this patent has a particle size ranging from 4-80 millimicrons. It is contemplated, however, that other forms of silica particles might be used, such as fumed or precipitated silica.
Another patent which involves the use of silica in battery construction is U.S. Pat. No. 4,317,872, issued Mar. 2, 1982 to Varma, entitled "Lead-Acid Battery with Gel Electrolyte". In this device, the electrolyte includes a silica component which is absorbed into the glass mat, the mat being made from micro fiber. Gelled electrolytes had been used in prior systems to eliminate spilling and the need for constant maintenance. Unfortunately, most prior gel systems did not have good electrical properties and the cycling characteristics did not compare well with batteries using liquid electrolytes. According to the patent, the gel could crack, allowing air to carry oxygen to the plates or electrodes of the battery, thereby allowing a quicker self-discharge than is desirable. This patent is directed to the combination of the glass mat technology previously described and gel technology, in which a gel electrolyte including silica components is employed. The separator material includes a silicate component integrally mixed with an oxygen compound of boron, forming a microfiber mat having a pore size between about 0.5-10 microns. In the preferred form, the separator material is in contact with and substantially envelops at least one of the electrodes. The patent discloses the use of precipitated silica, a finer silica typically having a very small particle size. When this material is combined with the other components of the electrolyte, it is absorbed into the separator.